Human Impacts on Owls Within the United States
Population Impacts : Habitat Alteration
Human alteration and destruction of habitat causes significant impacts on owls. Loss of habitat is the primary factor in the decline of raptor populations throughout the world. In addition to agriculture, logging, urbanization, recreation, energy and mineral development are altering owl habitats throughout the United States. In the United States, “98% of the tall-grass prairies have been plowed, half of the wetlands drained, 90-95% of old-growth forests cut, and overall forest cover reduced by 33% (Miller 1994).” Many species of owls have already begun to feel the tightening grip of altered habitats.
One way that owl habitat is altered is by urbanization and building of man-made structures. Urban areas have a population greater than 2,500. Approximately 75% of Americans live in urban areas and the remaining 25% live in rural areas (Miller 1994). This can be beneficial or harmful to owl populations. Owls could benefit from urbanization because it moves people into a condensed area and keeps them from being spread out. The city removes habitat in a small area, keeping other areas from being directly effected. Urbanization can also be harmful. Altering habitat drastically can make it unsuitable for most owl species. Some species such as Screech Owls and Great Horned Owls are more tolerant of urbanization, but most species are negatively impacted.
Some owl species tend to be habitat specialists. They have specific habitat requirements in which they need to survive. Pima County, Arizona is considering designating habitat for Northern Pygmy Owls (Glaucidium gnoma). In an article published in July 1999, there was concern by builders. The concern is that only 16% of the county is taxable. Designating 731,000 acres of habitat as critical for the Pygmy Owl could be harmful to the economy. If the land is designated, private landowners will need a federal permit to develop their land. In addition, taxes may have to be increased to compensate the loss of revenue. This in turn may result in fewer people being able to afford housing in the area. One person stated, “We have to ask, are we putting the owl before the people (Sodoma 1999)?” This is a controversial problem throughout the United States and will likely continue to be for some time to come.
In California 's Silicon Valley, the Burrowing Owl is being affected by development. Land prices in the area are high because of the computer industry, so developers are the only people able to purchase and develop the land. This alters Burrowing Owl habitat. There is an effort throughout the region to try and preserve as much habitat as possible to save the decreasing population of Burrowing Owls. For example, Mission College has implemented a plan to preserve some owl sites. They installed artificial burrows and keep the grass cut low which the owl prefers. Although conserving the Burrowing Owl habitat has been difficult, the region is in the developmental stages of implementing a plan to conserve the Burrowing Owls and their habitat (Holmes 1998).
The most well known habitat alteration that effects owls is logging. The impact of logging on each species varies. The method of harvesting trees can determine that impact. The four main harvesting methods are selective cutting, shelterwood cutting, seed-tree cutting, and clear-cutting (Miller 1994). Selective cutting is the removal of intermediate-aged or mature trees in an uneven-aged forest (Miller 1994). This harvesting style can be both beneficial and harmful to owls. It may be beneficial to owls that do not prefer dense forests and those that can live in both old and intermediate-aged forests. This style of harvesting does not remove all the trees and leaves the forest in an uneven aged state with young, middle aged, and mature trees. It may be detrimental to owls that prefer a specific age group of trees. Some owls such as Great Horned Owls are generalists. They are able to live in a variety of habitats. Others like the Spotted Owl are specialists and need old-growth forest to survive.
Shelterwood cutting is another form of logging. This style of harvesting involves removing the mature trees usually in two cuttings (Miller 1994). Most of the mature trees are removed in the first cut leaving some smaller trees to shelter the seedlings. When the seedlings are well established, the remaining trees are cut. This style of logging can have a drastic impact on owls that prefer mature or old-growth forest. The benefit of this logging style is that it does not remove all the older trees at once. They are removed over a period of time, which may be beneficial in allowing owls to adapt to the altered landscape more easily.
Seed-tree cutting involves the removal of nearly all the trees in one cutting (Miller 1994). A few of the best trees are left for producing seedlings. This style of cutting may be beneficial to species that do not require a developed forest because the trees are removed. The new forest is an even-aged forest. Spotted Owls and Great Grey Owls would most likely be harmed by such logging practices.
Clear-cutting removes all trees in a given area in one cutting. There are variations of clear-cutting such as strip logging and whole-tree harvesting. The cutting may be in a patch, strip, or a whole stand. In the United States, nearly 66% of the annual timber harvest is attained by clear-cutting (Miller 1994). The practice of clear-cutting has a negative impact on owls that depend on trees for nesting. Clear-cutting can also be beneficial. It creates foraging habitat. Many owls such as Great Grey Owls prefer to hunt along forest margins which provide open areas for visual searching for prey (Johnsgard 1988). Also, clear-cutting can create suitable habitat for owls that do not nest in trees such as the Burrowing Owl and the Short-Eared Owl.
Studies on the Spotted Owl, Great Gray Owl, and the Barred Owl are presented in the following section to illustrate the impact of human-altered habitat on owl populations. These species were chosen because of the abundance of information on them.
Spotted Owl : The Northern Spotted Owl has become the symbol in America with the Jobs/Logging vs. Owl debate. In a study by Forsman, Meslow, and Wight (1984), 98% of the sites with Spotted Owls were either old-growth forests or a mixture of mature and old-growth forests. They also prefer to live in multi-layered canopies and uneven-aged forests. The Spotted Owls do not build their own nest. They utilize natural sites of broken tree tops and cavities (Beck and Gould 1992). The average home ranges of Spotted Owls range from 400 to 2776 hectares (Johnsgard 1988). The main habitat alteration that seems to effect Spotted Owls is the logging of old-growth forest in the western United States (Forsman and Bull 1989). The concern for the Spotted Owl began back in the 1970's when the logging vs. habitat debate in the Pacific Northwest was on the rise (Beck and Gould 1992). Some studies have indicated that Spotted Owl populations have decreased with the elimination of old-growth forests and the replacement by younger forests. For many decades throughout the 20 th -century, the Pacific Northwest was heavily logged because of the ever increasing need for timber in the United States. Much of the old-growth forests in the United States have vanished, but most of what remains is located in the Northwest. Less than 10 percent of the old-growth forests that once stood in the Northwest are still standing. Many people in the Northwest depend on logging for employment, so it is understandable that there is much opposition to the preservation of old-growth forests for Spotted Owls (Gup 1990). Although there have been some concerns about the effect that preservation of owl habitat may have on jobs and the economy, the economy in Oregon is currently on the rise. In 1993, President Clinton's forest plan caused many job losses due to lumber trade regulations, but it included occupational retraining. Timber harvesting is not the only reason for concern with the Spotted Owl. The California Spotted Owl is also losing habitat for breeding and foraging. New residential developments, fires, recreational activities, and the “mining” of water from streams in the owl's habitat are impacting the California Spotted Owls population (Beck and Gould 1992). This species of owl will continue to be one of concern in the United States because of their dwindling numbers.
Great Grey Owl : The Great Grey Owl prefers mature or old-growth forests throughout the Pacific Northwest. They nest in abandoned nests or on top of tree trunks. Logging effects on Great Grey populations can either be beneficial or harmful. Depending on the extent, type and time of logging, it can either benefit or be detrimental to the species (Johnsgard 1988). “In northeastern Oregon, owls spend the majority of their time foraging in selectively logged stands”, and in southeastern Idaho, the owls prey on gophers in clear-cut areas (Forsman and Bull 1989). The reduction of dead trees used for nesting, and dense canopy trees used by juveniles are problems that logging causes with Great Gray Owls.
Barred Owl : The Barred Owl live primarily in mature forests and swamps. They nest in tree cavities but sometimes will use abandoned nests. Logging of mature and old-growth forests has been the primary habitat alteration that affects this species (Smith et al. 19__).
In Connecticut and New Hampshire, Barred Owls avoid developed areas. However, there is evidence that Barred Owls are able to sustain human development more readily than other owls (Smith 1975).
Although there are many other species of owls, this is a sample of some of the impacts caused by habitat alteration and destruction.
One of the beneficial impacts humans have on owls is through legal protection. In the United States, there are serious penalties for harming all birds of prey including owls. Legislation has helped to increase public awareness, and reduce shootings, poisoning, and habitat destruction (Postovit 1987).
An early law, the Migratory Bird Treaty Act of 1918, prohibited the shooting of all raptors. Though it took many years for the public to become conscious of the act, it has had a positive impact on owls and other raptors throughout the United States (Postovit 1987).
In 1973, the United States Congress approved the Endangered Species Act. This law restricts the possession of any endangered or threatened species in the United States. Not only does this include species found throughout the United States, but also those found world wide (Weidensaul 1996).
The ban on DDT was an extremely important piece of legislation. This chemical has had an impact on raptor species reproduction throughout the United States since its use began during WWII (Postovit 1987). Without this federal legislation, the populations of owls throughout the nation would be far more negatively impacted by deaths. Further legislation is needed to protect owls from habitat alteration, poisoning, and other negative human impacts.
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